The present application relates to a new functional molecular element that changes in conductivity under the influence of electric field, a method for production thereof, and a functional molecular device.
There has recently emerged a new technology called nanotechnology to observe, produce, and utilize microstructure of the order of 10−8 m =10 nm.
The scanning tunneling microscope, which is an extremely high-accuracy microscope invented in the late 1980's, has made it possible to observe as well as manipulate atoms and molecules individually.
In fact, writing letters with atoms arranged on the crystal surface has been reported. Despite such capability, it is not practical to create a new material or to assemble a new device by manipulating an immense number of atoms and molecules individually.
Making an extremely small structure on the order of nanometers by manipulating atoms or molecules or groups thereof individually requires a new technology for ultraprecision fabrication. There are two systems suitable for microprocessing with accuracy of the order of nanometers.
One is so-called top-down system which has been used to fabricate various semiconductor devices in related art. It is exemplified by fabrication of integrated circuits from silicon wafers by extremely accurate etching to the very limits. The other is so-called bottom-up system, which is designed to assemble a desired nanostructure from atoms and molecules as extremely small constituents.
The limit of size of nanostructure that could be achieved by the top-down system is suggested by Moore's law (proposed in 1965 by Gordon Moore as a co-founder of Intel Corporation) which stipulates that the number of transistors on a chip will be doubling every 18 months. In fact, the rate of integration of transistors has increased over the past 30 years from 1965 as predicted by Moore's law.
It is considered that Moore's law will continue to be effective according to the International Technology Roadmap for Semiconductor (ITRS) 2005 Edition for coming 15 years which was published by the Semiconductor Industry Association (SIA).
The ITRS consists of a near-term roadmap up to 2013 and a long-term roadmap up to 2020. The former predicts that in 2013 the DRAM half pitch will be 32 nm. The latter predicts that the DRAM half pitch will shrink to 14 nm in 2020.
As the degree of microfabrication increases further, resulting semiconductor chips run faster with less power consumption. Moreover, improved microfabrication yields more products from a single wafer, thereby reducing production cost. This is the reason why microprocessor makers compete in the process rule for new products and the degree of transistor integration.
In November 1999, a research group in the United States published an epoch-making result of research on the technology of microfabrication. This technology was developed by Prof. Chenming Hu and his team in charge of computer science in the University of California at Berkeley. Its object is to design the gate on a field effect transistor (FET) called FinFET. It makes it possible to form 400 times as many transistors as before on a single semiconductor chip.
The gate is an electrode to control the flow of electrons in the channel of an FET. According to the existing ordinary design, it is placed parallel to the surface of the semiconductor so that it controls the channel from one side. The disadvantage of this gate arrangement is that the gate cannot cut off the flow of electrons unless it is longer than prescribed. Therefore, the gate length has been considered to be one factor that restricts miniaturization of transistors.
By contrast, the FinFET has a fork-shaped gate which holds the channel between both sides for effective channel control. This structure permits further reduction of gate length and further miniaturization of transistors than the one in related art.
The above-mentioned research group produced a prototype FET having a gate length of 18 nm, which is one-tenth of the existing ordinary gate length. This achievement corresponds to the size predicted for the year 2014 in the long-term roadmap of ITRS. Prof. Chenming Hu and his team do not claim patent for their invention in hope that their new technology will be widely accepted in the semiconductor industry. There is the possibility of FinFET becoming the mainstream of the manufacturing technology.
On the other hand, experts predict that the time will soon come when Moore's law is no longer valid in the light of natural rule.
Since the current major technology for semiconductor chips involves lithography to form circuit patterns on silicon wafers, it is necessary to improve resolution for further miniaturization. To this end, it is necessary to develop a practical technology to employ light with a shorter wavelength.
Another problem involved in increasing the degree of integration is excessive heat evolution, which leads to malfunction and thermal breakage at high temperatures.
Moreover, there are some experts who predict that miniaturization of semiconductor chips that continues at the present pace will reach the stage in which equipment cost and process cost go up and yields go down. As the result, the semiconductor industry will not pay in around the year 2015.
Another problem which has been pointed out recently is line edge roughness (or minute irregularities around pattern edges). As to irregularities on the surface of a resist mask, it is said that the size of molecules constituting a resist and the distance of diffusion of acid in a chemically amplified photoresist become critical as the pattern size is reduced more than before. Another important problem is that the device characteristics depend on the cycle of irregularities on the pattern edge.
To resolve the above-mentioned bottleneck in the top-down system, a new technology is under development which makes individual molecules function as electronic parts and constructs electronic devices, such as molecular switches, from individual molecules by the bottom-up system.
Researches on the technology to make nanostructure from metal, ceramics, or semiconductor by the bottom-up system are also under way. It would be possible to design and create (molecular) devices entirely different from ones in related art if millions of species of diverse molecules independently varying in size and function are assembled by the bottom-up system.
Conductive molecules, each having a width of only 0.5 nm, which is by far smaller than the line width of about 100 nm achieved in the current IC technology, carry a potential for thousands times high-density wiring. Moreover, it would be possible to realize a recoding device with a capacity larger than 10,000 times that of DVD if individual molecules are used as memory elements.
Molecular devices are chemically synthesized unlike silicon semiconductor devices in related art. The world's first organic transistor of polythiophene (polymer) was developed in 1986 by Hiroshi Koezuka of Mitsubishi Electric Corporation.
Further, research groups in Hewlett-Packard Company and the University of California at Los Angeles developed organic electronic devices and published them in the July 1999 issue of Science and applied for patent. (See in U.S. Pat. No. 6,256,767B1 and U.S. Pat. No. 6,128,214). They also made switches from molecular film composed of millions of rotaxane molecules (which are organic molecules) and completed AND gates (as fundamental logic circuits) by joining them together.
Joint research groups in Rice University and Yale University in the US created a molecular switch which performs switching action as the molecular structure changes upon electron injection in an electric field. They published it in the November 1999 issue of Science. (See J. Chen, M. A. Reed, A. M. Rawlett and J. M. Tour. “Large on-off ratios and negative differential resistance in a molecular electronic device”, Science, 1999, Vol. 286, 1552 to 1551) It has the repeating on-off function that was not achieved by the research groups in Hewlett-Packard Company and the University of California at Los Angeles. In addition, it has one millionth of the size of ordinary transistor, and this smallness will contribute to small high-performance computers.
Prof. J. Tour (of Rice University, chemistry) who succeeded in synthesis suggests that the cost of molecular switches would be only one thousandth of that of semiconductors in related art because expensive clean rooms are not necessary for their production. He is planning to construct a hybrid computer (composed of organic molecules and silicon) within five to ten years.
An organic thin-film transistor was completed from pentacene single crystals in Bell Labs (Lucent Technologies Inc.) in 1999.
The molecular devices with functions of electronic components which have been studied extensively so far are limited mostly to those which are driven by light, heat, protons, or ions (as described in “Molecular Switches” compiled by Ben L. Feringa, WILEY-VCH, Weinheim, 2001) and they merely include a few examples that are driven by electric field.
The above-mentioned problem of line edge roughness still remains in the molecular device, and it will become serious as the pattern is miniaturized further. One common way to avoid this problem in molecular devices is to introduce a thiol group into the terminal of the molecule for direct connection to a gold electrode. (See M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin and J. M. Tour, “Conductance of a molecular junction”, Science, 1997, Vol. 278, 252 to 254) The molecular devices are superior in reproducibility to those of inorganic material because they are composed of minimum units smaller than roughness.
However, electrical connection between the thiol group and the gold electrode poses a problem with large electric resistance no matter whether the molecule itself has good electrical properties, and this prevents the molecular device from improving in its characteristic properties. (See J. M. Wessels, H. G. Nothofer, W. E. Ford, F. von Wrochem, F. Scholz, T. Vossmeyer, A. Schroedter, H. Weller and A. Yasuda, “Optical and electrical properties of three-dimensional interlinked gold nanoparticle assemblies”, Journal of the American Chemical Society, 126(10), 3349 to 3356, Mar. 17, 2004)
Most of molecular elements in related art which are driven by electric field work in such a way that the constituent molecules under the influence of electric field change in their electronic state, which in turn changes conductivity between two (or more) electrodes. In the case of organic field effect transistor (organic FET), for example, the electric field that acts on organic molecules in the channel region changes the carrier movement in the organic molecules. The operating characteristics of the molecular element are greatly affected by the contact resistance between the constituent molecules and the electrode which is very large as mentioned above.
One of the present inventors has proposed a functional molecular element which works as a molecular switch to turn on and off electric current as the molecular structure changes under the influence of electric field. However, it is apparent that even the functional molecular element based on such a new principle cannot escape its operating characteristics being affected by the large contact resistance between the constituent molecules and the electrode.
Minimizing contact resistance between the organic molecules and the electrode is necessary even in the case where a molecular layer to flow electrons is interposed between opposing electrodes, as in solar cells.
Therefore it is desirable to provide a functional molecular element with a new construction which reduces contact resistance between the constituent molecules and the electrode, and a method for production thereof, and a functional molecular device.